Summary: Facial clefts are one of the most common birth defects, affecting 2 out of every thousand babies. While the main causes of facial clefts are unknown, it is obvious that genetics plays a strong role. We've shown that families with one member affected by clefts are 40 times as likely to have a baby with a cleft. Environmental factors are also presumed to play a role in clefts. For example, clefts are easily produced in experimental animals exposed to teratogens. It is likely that humans vary in their genetic susceptibility to teratogens that cause clefts. More than a decade ago, we anticipated that genetic susceptibility would become an important area of epidemiologic research at NIEHS. Accordingly, we selected facial clefts as a condition with both genetic and environmental causes, and we began in 1992 to develop a study to address the causes of clefts. In 1996 we launched a population-based case-control study of facial clefts in Norway. (Norway has one of the highest rates of cleft lip and palate in the world.) The field phase was completed in 2002. We enrolled 88% of all babies with facial clefts born in Norway between 1996 and 2002 (574 cases), and 76% of eligible control infants (763) selected randomly from the population. Mothers of these infants provided detailed information on occupational and other exposures during pregnancy, as well as on nutrition, personal habits and medical history. Biological samples for DNA analysis were collected from cases and controls as well as their mothers, fathers and siblings. These total nearly 4000 people. DNA has been extracted from these samples and is now ready for genetic analysis. In the course of carrying out this study, we developed and published a new statistical strategy for the analysis of genetic data in case-parent triads that has been widely adapted. We have demonstrated the application of this new method in a preliminary analysis of 262 case-parent triads. We are working intensively on analyses of folic acid, cigarette smoking and hazardous occupations, and their effects on the risk of facial clefts. In our most recent analyses we have demonstrated a strong dose-response association between maternal cigarette smoking and facial clefts. In order to pursue a more biologically based understanding of this association, we completed DNA analysis of eleven genes associated with various aspects of cigarette metabolism, and searched for evidence of genetic susceptibility. We found no evidence that these metabolism genes play a role in the smoking effect on clefts. In a similar effort, we explored the effect of genes involved with the metabolism of folic acid. We recently showed that folic acid supplementation reduces the risk of cleft lip with or without cleft palate, although not the risk of cleft palate alone (which is a distinct birth defect in its embryologic development). We assessed nine genes that are involved in the metabolism of folic acid, in order to see whether the effect of folic acid supplementation may be stronger among certain groups of metabolically susceptible women. We found no evidence of such susceptibility. The lack of evidence for gene-environment interaction in these two analyses points to the difficulties of identifying such interactions, and the need to extend genetic analyses to a much wider range of genes. We are currently participating in collaborative efforts to look at more than 1000 SNPs from candidate genes, and thereafter to explore whole genome scans of these biological samples.